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An Overview of Surfactant Science and Technology

Continuing rapid changes in our modern, technology-oriented society continues to make it increasingly important for scientists, engineers, regulators, managers, and end users to remain abreast of the latest in the technologies impacting their work. While digital electronics gets the lion's share of the general press, good old-fashioned chemistry remains vitally important for a myriad of manufacturing, processing, and functional operations. Cleaning, lubricating, adhesion, surface release, water repellency, and many more very basic surface and interfacial phenomena remain essential to the proper functioning of the newer technological processes and products.

The scientific and technical journals published worldwide number in the thousands, and this number seems to increase yearly when we consider the electronic journals, prepublication sites, etc. Paralleling the proliferation of scientific literature in general has been an increasing divergence into fields of "pure" science, i.e. studies with their principal goal being a general advancement of basic human knowledge, and "applied" science and technology, in which the research is driven by some anticipated or hoped-for application or solution. Few areas of chemistry have exhibited this growing dichotomy of purpose more than the study of surface and colloid science, especially as applied to surface activity and surface-active materials or surfactants. Even the nomenclature used in discussing materials showing surface activity is varied, depending on the context of the discussion. It is not surprising, then, that the world of surface activity and surface-active agents can, at times, seem complex and confusing to those not intimately involved in it on a day-to-day basis.

When one considers the impact of surface science in general, and emulsions, dispersions, foaming agents, wetting agents, etc., specifically, on our day-to-day existence, the picture that develops reveals the great extent to which these areas of chemistry and chemical technology permeate our lives. From the fundamental aspects of biological membrane formation and function in living cells to the more "far-out" problems related to the behavior of liquids under "unusual" conditions such as high temperature and pressure environments or under low gravity conditions, as for example, how liquids wet the walls of a rocket's fuel tank in a low gravity environment, the physical chemistry of the interactions among various phases at interfaces lies at the root of much of our modern lifestyle.

Industrial concerns, whose very lifeblood may be intimately linked to the application of the basic principles of interfacial interactions, often ignore the potential benefits of fundamental research in these areas in favor of an empirical trial-and-error approach, which may lead to a viable process but one that could possibly be better understood and even significantly improved by the application of more "fundamental" science. In many cases, the prevailing philosophy seems to be, to paraphrase an adage, "A dollar in the hand is worth two in the laboratory." Unfortunately, such an approach often results in more dollars down the drain than many management-level decision makers care to admit. Academic researchers, on the other hand, are sometimes guilty of ignoring the potential practical aspects of their work in favor of experimental sophistication and the "Holy Grail" of the definitive theory or model, although the new thinking in academics of "patent everything, just in case" has brought to light some new potential uses of surfactant technology. Neither philosophy alone truly satisfies the needs of our technological existence. Each approach makes its valuable contribution to the overall advancement of human knowledge; however, it sometimes appears that a great deal is lost in the communication gap between the two.

The science and technology of surfactants have possibly suffered a double whammy from the functional divergence of academic and applied research. Academic interest in surfactants, while increasing in recent years, has generally concentrated on highly purified, homogeneous materials, quite often limited to a few compounds such as the classic examples of sodium dodecyl sulfate (SDS) or cetyltrimethylammonium bromide (CTAB) and elegant analytical techniques. While providing a wealth of useful information related to the system under investigation, the application of such information to more complex practical materials and processes is often less than obvious and is sometimes misleading. In the results-oriented industrial environment, with some significant exceptions, surfactant research is often carried out on a "make it work and don't worry about why!" basis. The industrially interesting materials are usually complex mixtures of homologues and isomers or contain impurities resulting from chemical side reactions, unreacted starting materials, solvents, etc. Particularly significant surface property changes can be induced by the presence of such impurities as inorganic salts or long-chain alcohols or other molecules remaining after processing. While the presence of such impurities and mixtures will often produce superior results in practice, analysis of the process may be difficult because of the unknown or variable nature of the surfactant composition. Considering the limitations imposed by each "school" of surfactant research, it is not surprising to find that a practical fusion of the two approaches can be difficult to achieve.

The different views of surfactant science and technology have spawned their own distinctive terminologies and literatures. While the academic or fundamental investigator may probe the properties of surface-active agents, surfactants, tensides (a very old and seldom used term today), or amphiphiles, the industrial chemist may be concerned with the characteristics of soaps, detergents, emulsifiers, wetting agents, etc. The former group may publish their results primarily in the Journal of Physical Chemistry, Colloids and Surfaces, Langmuir, or the Journal of Colloid and Interface Science and the latter in the Journal of the American Oil Chemists' Society, the Journal of Dispersion Science and Technology, or one of the other technologically specialized publications aimed at specific areas of application (foods, cosmetics, paints, etc.). All too often, the value of the results to each community can become lost in the sea of manuscripts and the philosophical and operational gulf that sometimes develops between the two.

Before beginning a discussion of specific aspects of the chemistry of surface-active materials and surfactant action, it may be useful to have some idea of the history of surfactants and how their synthesis and use have evolved through the years. Because of parallel developments in various areas of the world, the secrecy of industrial research efforts, and the effects of two world wars, the exact details of the evolution of surfactant science and technology may be subject to some controversy regarding the specific order and timing of specific developments. In any case, the major facts are (hopefully!) correct.

1.1 A Brief History of Surfactant Science and Technology

The pedigree of what are sometimes referred to as "synthetic" surfactants is well documented, unlike that of some of the older "natural" alkali soaps and other plant- or animal-derived materials found to be useful centuries ago by trial and error and that are still used today. However, it is not easy to pinpoint the exact time when the synthetic surfactant industry came into being. In a strictly chemical sense, a soap is a compound formed by the reaction of a fatty acid that is of limited solubility in water with an alkali metal or other organic base to produce a carboxylic acid salt. The resulting salt will exhibit enhanced water solubility sufficient to produce a significant level of surface activity. As a result, soaps were found to have important uses for cleaning purposes. However, those uses can be severely limited by the specific conditions of use, such as temperature, pH, the type and concentration of salts present in their aqueous solutions, etc. Since the traditional soaps are limited in their scope of beneficial surfactant properties for most modern applications, they or newer materials based on petrochemical or other feedstocks require some form of chemical modification to enhance their water solubility and make them more useful in most modern industrial and technological applications. In fact, it is not uncommon to see them referred to as a separate category of materials from other surface-active species as in "soaps and surfactants" or "soaps and detergents." As already mentioned, it is common today to see some surface-active materials referred to as "natural" and others as being "synthetic" or "chemical" materials. Any such distinction is, of course, a purely artificial distinction since even the traditional soaps require chemical modification to produce useful surface activity and the most synthetic detergents can eventually trace their heritage back to natural sources. Nevertheless, the following discussions will use the natural versus synthetic distinction to "divide the waters," so to speak. More detailed information on the divide between natural and synthetic surfactants...